Can-making steel sheet

ABSTRACT

There is provided an ultra-thin and high strength three-piece can-making plated steel sheet which is excellent in weldability and flange forming processibility containing C ≦ about 0.004 wt %, Si≦ about 0.03 wt %, Mn: about 0.05-0.6 wt %, P≦ about 0.02 wt %, S≦ about 0.02 wt %, N≦ about 0.01 wt % Al: about 0.005-0.1 wt %, Nb: about 0.001-0.1 wt % and the balance of inevitable impurities. The steel sheet has a maximum recrystallization grain size of 30 μm or less and a ratio of area of 50% or more which is occupied by recrystallization grains of about 5-25 μm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.08/163,950 filed Dec. 8, 1993, now abandoned, which is a continuation ofU.S. application Ser. No. 08/043,189 filed Apr. 6, 1993, now U.S. Pat.No. 5,360,676.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a steel sheet used for makingthree-piece cans and a method of making the steel sheet, and morespecifically, to a method of manufacturing a thin and high strengthcan-making steel sheet excellent in flange formability and weldability.

2. Description of the Related Art

Can-making steel sheets are required to have a thin thickness-and highstrength for reducing the cost of making cans. In addition to the above,three-piece can-making steel sheets are further required to be excellentin high speed weldability and flange formability after welding. Thisoccurs because of the following reasons: When three-piece cans are made,can barrels used to form the cans are made by high speed electricwelding at a speed up to 70 m/min. Moreover, after the can barrels havebeen made, a flange forming step is carried out on the can barrels suchthat each barrel contains a welded portion to mount upper and bottomlids.

However, the prior art has had the problem that when the thickness of asheet is reduced, the proper welding range is narrowed. Thus, when thewelding current is increased, splashing is caused, the weld is hardenedand flange cracking tends to occur at the HAZ (heat affected zone) ofthe weld in a formed flange after formation of a can barrel.

Further, in a recent can-making process, a coil-coated steel strip,which was previously coated at the stage of a coil or a film-laminatedsteel strip which is made by bonding a printed film to a steel sheetcoil, was employed to improve the efficiency of the coating process. Inthose methods, so-called non-varnished portions to which no coating hasbeen applied or no film has been laminated are preferably formedparallel to the rolling direction when a can is made in order toeffectively carry out slitting. However, problems arise in that when thenon-varnished portion of a sheet prepared as described above is welded,and then flange forming is carried out, cracking is liable to occur atthe HAZ. To cope with this problem, the non-varnished portions areconventionally formed in a direction perpendicular to the rollingdirection. Thus, a coil-coated steel strip and a film-laminated steelstrip cannot be effectively manufactured.

Many efforts have been made to solve these problems. However, noproposals for directly solving these problems has been found forcan-making steel sheets. For example, although Japanese Patent ExaminedPublication No. 1-52450 discloses a method of manufacturing a can-makingsteel sheet T1-T3 by continuously annealing ultra-low carbon steel andthen temper-rolling the same, it does not disclose a solution to theabove problems.

Although U.S. Pat. No. 4,889,566 discloses an ultra-low carbon steelsheet added with Ti, Nb, B to improve spot weldability ofautomobile-making steel sheets, the patent does not provide anysuggestion with respect to problems characteristic to three-piececan-making steel sheets.

Although U.S. Pat. No. 5,156,694 discloses a method of adding Nb, B andfurther Ti to an ultra-low carbon steel to improve press formability,deep drawability and further fatigue resistant strength at a weld of anautomobile-making steel sheet, it mentions nothing with respect to thewelding characteristics of can-making steel sheets. Further, the patentmentions nothing with respect to the formability of the HAZ portion of aweld.

OBJECTS OF THE INVENTION

An object of the present invention is to provide a novel three-piececan-making steel sheet and in particular to provide a three-piececan-making steel sheet which is excellent in high speed weldability--itdoes not cause HAZ cracking of a weld in flange processing (hereinafter,simply referred to as flange cracking or HAZ cracking) carried out afterformation of a can barrel.

Another object of the present invention is to provide a can-making steelsheet which enables a can barrel to be wound parallel to the rollingdirection and further to which high speed welding can be carried out tothereby provide a steel sheet from which a film-laminated steel stripcan be made in which a so-called non-varnished portion is formed in therolling direction of the steel strip.

A further object of the present invention is to provide a method ofmanufacturing a thin and high strength can-making steel sheet having theabove characteristics.

Other objects of the present invention will be apparent from the claims,detailed description and the like thereof.

SUMMARY OF THE INVENTION

The present invention provides an ultra-thin and high strengthCan-making steel sheet which is excellent in weldability and flangeformability and contains C≦ about 0.004 wt %, Si≦ about 0.03 wt %, Mn:about 0.05-0.6 wt %, P≦ about 0.02 wt %, S≦ about 0.02 wt %, N≦ about0.01 wt %, Al: about 0.005-0.1 wt %, Nb: about 0.001-0.1 wt % and abalance of inevitable impurities and has a maximum recrystallizationgrain size of about 30 μm or less and a ratio of area of about 50% ormore which is occupied by recrystallization grains of about 5-25 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the ratio ofoccurrences of HAZ cracking and maximum grain size;

FIG. 2 is a graph showing the relationship between total thickness of aweld and the ratio of occurrences of HAZ cracking; and

FIG. 3 is a schematic view of a weld when a can-making steel sheet isformed into a can.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to studies by the inventors, when a high strength steel isused and non-varnished portions are formed in the L direction, HAZcracking often occurs during flange forming under the followingconditions. First, HAZ cracking is liable to occur when a large stressconcentration is produced by restriction of the nugget portion of theweld. More specifically, when the nugget portion of the weld has a largepeak hardness, a stress concentration occurs due to the restriction ofthat portion and cracking occurs at the HAZ portion in the vicinity ofthe nugget portion in combination with a small elongation of the steelsheet in the C direction and the weakness of the grain boundary to beshown later. When a sheet is rolled into a thin thickness, thisphenomenon is made more remarkable because its elongation is made small.

Further, when the grain size of the sheet is larger than a certainvalue, HAZ cracking occurs. More specifically, when the grain size islarge, the boundary of a pancake-shaped grain is weakened and the HAZportion is liable to be broken by a tensile strength applied in adirection perpendicular to the longitudinal direction of the grainboundary when the flange forming step is carried out.

FIG. 3 is a schematic view of a weld when a can-making steel sheet isformed into a can. In FIG. 3, a sheet to be formed 1 is overlapped atthe opposite ends thereof and the overlapped portion is welded byapplying a current while the overlapped portion is pressed by electroderings 2, 3 through an external copper wire 5 and an internal copper wire6 to thereby form a nugget portion 4. Although the weld of a can barrelis formed while being overlapped, the pressure of the overlapped portionis not reduced by the press carried out by electrode rings 2, 3 when theoverlapped portion is welded and a stress concentration is caused by therestriction of the overlapped portion when the sheet to be processed 1has a thickness larger than a certain value. Thus, cracking occurs inthe aforesaid weak grain boundary.

The inventors have succeeded providing a novel three-piece can-makingsteel sheet by making clear the material conditions of a sheet to beprocessed with respect to HAZ cracking which occurs when flange formingis carried out.

(1) Chemical Composition

C generally hardens a material and in particular rapidly increases thehardness of the HAZ portion. It also affects flange formingcharacteristics through the hardness of the HAZ portion of the weld of acan-making steel sheet and the thickness of the weld. Therefore, C islimited to a content of about 0.004% or less.

A conventional can-making steel sheet has a C content of 0.03-0.06%. Inthis case, when a sheet has a thickness of about 0.2-0.25 mm,non-varnished portions are formed perpendicular to the rolling directionand welding is carried out along the non-varnished portions, no HAZcracking occurs. When the non-varnished portions are formed parallel tothe rolling direction and welding is carried out along the non-varnishedportions, flange cracking occurs when a can is expanded.

This phenomenon is believed to be caused by an increase in the hardnessof the weld and stress concentration due to an increase in the thicknessof the weld caused by the increase in hardness. These phenomena can besolved by reducing the C content to a very low level, specifically to alevel of about 0.004% or less. For example, when C is contained in anamount of about 0.004% or less, the weld nugget portion has a lowhardness even if it is rapidly cooled when welded because the grain sizeis not made small, the stress concentration is eased at the HAZ portionand it is therefore difficult to cause HAZ cracking (compare Example 1of the invention with Comparative Example 3 in Table 1).

An ultra-low carbon cold-rolled steel sheet has a large grain size andis in a softened state when it has been subjected to a CAL (continuousannealing). When the steel sheet is used as a can-making steel sheet,however, it is subjected to temper-rolling after having been annealed sothat its strength is increased by work-hardening. Thus, the stressconcentration is eased because the steel sheet is easily softened byheat applied during welding and greatly elongated. This also occursbecause the thickness of the portion overlapped for welding of thenugget portion is made thin by pressure applied during welding with theresult that the steel sheet is softened during welding and norestriction is caused by the thickness of the sheet.

When C is contained in an amount of about 0.004% or less, however, thegrain size of a steel sheet is generally increased. Thus, when a can isexpanded, flange cracking occurs at the welded HAZ portion, to which aseparate countermeasure must be applied.

Si not only deteriorates the corrosion resistance of can-making steelsheets but also extremely hardens the material and, as a result,adversely affects flange forming characteristics. Thus, Si must becontained in an amount of about 0.03% or less.

When Mn is contained in an amount of about 0.6% or more, the grain sizeis excessively refined, the material is hardened and the total thicknessof the nugget portion is made too thick, thereby deteriorating flangeforming characteristics. On the other hand, although Mn has no lowercontent limit from the view point of flange forming characteristics, itmust be added in an amount of about 0.05% or more to prevent-edgecracking of a hot-rolled coil caused when the steel sheet is in therolling process.

Since P hardens a material and deteriorates the corrosion resistance ofcan-making steel sheets, it must be contained in an amount of about0.02% or less.

Although S does not directly affect flange forming characteristics, itis limited to a content of about 0.02% or less to prevent edge crackingof the hot-rolled steel sheet in the steel sheet manufacturing process.It should be noted that S is preferably contained in such an amount asto set a Mn/S ratio to about 8 or more in the relationship thereof withthe aforesaid Mn to prevent edge cracking.

Al is generally added as a deoxidation agent in the steel manufacturingprocess to increase the cleanliness of the steel. Further, since Alfixes N in the steel sheet, it is added in an amount of about 0.005% ormore. Since excess addition of Al restricts the growth of crystal grainsand an adverse effect appears in flange forming characteristics,however, it is limited to an amount of about 0.10% or less.

Since an excess amount of N hardens steel to thereby deteriorate flangeforming characteristics, it is limited to an amount of about 0.01% orless.

Since O forms an oxide together with Al and Mn in steel and deterioratesthe corrosion resistance of a can and flange forming characteristics, itmust be limited to an amount of about 0.01% or less.

Nb is an important element which affects recrystallized grains havingbeen annealed and flange forming characteristics through therecrystallized grains. More specifically, although ultra-low carbonsteel is employed as the steel of the present invention from the viewpoint of hardness, thickness and the like of a weld as described above,Nb must be added to the ultra-low carbon steel to prevent coarsening ofcrystal grains which are characteristic of the ultra-low carbon steel tothereby prevent HAZ cracking. Nb is preferably added in an amount bywhich the grain size to be indicated later can be obtained, andspecifically it is preferably added in an amount of about 0.003-0.02%.

Although Sn, Sb, As and Te are not particularly related to the canexpanding properties which are the subject matter of the presentinvention, Sn and/or Sb may be added in an amount of about 0.001% orless, As may be added in an amount of about 0.001% or less and Te may beadded in an amount of about 0.0001% or less from a view point ofcorrosion resistance.

Ca may be added in an amount of about 0.005% or less for the shape ofinclusions such as Al₂ O₃, etc.

(2) Grain Size

An excessively large or excessively small grain size causes flangecracking at the HAZ portion in the can expanding process to be carriedout after the can barrel has been welded.

FIG. 1 shows the relationship between the maximum grain size of thecan-making steel sheet and flange cracking when the can barrel is woundparallel to the rolling direction of the steel sheet in place of aconventional direction perpendicular to the rolling direction of thesteel sheet. It can be found from FIG. 1 that when the can barrel iswound in the direction parallel to the rolling direction of the steelsheet, a lot of HAZ cracking typically occurs unless the maximumcrystallized grain size is about 30 μm or preferably about 25 μm orless. Although the reason for this phenomenon is not completely clear,it is believed to be caused by the weakness of the grain boundary due tothe segregation of impure elements at the grain boundary.

FIG. 2 shows the relationship between the degree of reduction of weldthickness and the ratio of occurrence of HAZ cracking when the canbarrel of a three-piece can is joined by high speed welding. As shown inFIG. 2, when the total thickness of the weld is 1.4 times or more thethickness of the sheet (hereinafter the thickness of the sheet is freelyabbreviated to "t") to be processed, stress produced in the flangeforming step is liable to concentrate. Thus, a good quantity of HAZcracking is liable to occur.

The grain size of the sheet to be processed affects the total thicknessof the weld. According to experiments carried out by the inventors, itwas found that when the ratio of area occupied by crystal grains of 5-25μm is about 50% or more, the total thickness of the weld is about 1.4times or less the thickness of the sheet to be processed when the usualhigh speed welding is carried out. It should be noted that since thedegree of winding and tightening processing of 7% or more is requiredfor cans such as beverage cans having a small can diameter, the minimumcrystal grain size must be about 10 μm or less in this case.

To summarize the aforesaid points, a sheet to be plated is required tohave the conditions that all the crystal grains exist in the range ofcrystal grains of about 30 μm or less and the ratio of area occupied bycrystal grains of about 5-25 μm is about 50% or more. In making thiscalculation, grain size is measured by a method of calculating averagevalues of the sizes in a longer diameter direction and shorter diameterdirection of the crystal grains in a sheet to be plated.

(3) Rolling Conditions

A steel sheet having the aforesaid distribution of grain sizes hasexcellent properties for a three-piece can-making steel sheet and asteel sheet having such a distribution of grain sizes and can bemanufactured by rolling a steel sheet satisfying the above amounts of Cand Nb under the following conditions.

(i) Finish Temperature in Hot-Rolling (FDT):

When the finishing temperature is too high or too low, grain sizes afterrecrystallization are unnecessarily coarsened. Thus, the temperature isset to 800°-900° C.

(ii) Coiling Temperature (CT)

When the coiling temperature is too high, crystal grains are grown andcoarsened by self-annealing, whereas when the temperature is too low,not only are crystal grains not sufficiently grown and made too smallbut also a rolled grain structure may remain even in an ultra-low carbonsteel. Thus, the CT is set to 500°-650° C.

(iii) Rolling Reduction in Cold-Rolling

The rolling reduction in cold-rolling is quite important as a factor forcontrolling a grain size. When the rolling reduction is too low, crystalsizes are coarsened in an annealing process to be carried out thereafterand the uniformity of the grain sizes is also lowered. Thus, the rollingreduction must be set to 85% or more. A hot-rolled steel strip ispreferably a little thicker to make the temperature distribution thereofuniform and the grain size resulting therefrom uniform. Thus, therolling reduction is preferably set to 90% or more to obtain acold-rolled steel sheet with a predetermined thickness from such aslightly thicker hot-rolled steel strip.

(iv) Continuous Annealing Conditions

The continuous annealing conditions are set according to a usual methodsuch that the temperature is 720° C. or more, which is therecrystallization temperature. The annealing time is within 60 secondsat 800° C. which is a temperature at which grain sizes are notcoarsened. These conditions are set to obtain a steel sheet having asuitable thickness.

EXAMPLES

Steels having compositions shown in Table 1 were melted in an amount of270 tons in a bottom-blowing converter and subjected to an R - H vacuumdegassing processing to obtain melted steels containing C decarbonizedto 0.004% or less, and slabs having required compositions shown in Table1 were manufactured by a continuously casting machine. These slabs eachhaving a size of 260 mm were rolled to hot-rolled sheets of 2.3 mmthickness under conditions shown in Table 1 and rolled to sheets of 0.22mm thick by a six-stand tandem cold-rolling mill after descaling. Therolling reduction in cold-rolling was 90.4%.

After being subjected to the cold-rolling process, the cold-rolledsheets were subjected to continuous annealing and further totemper-rolling so that they were rolled to ultra-thin steel sheets of0.130 mm thick. The rolling reduction in temper-rolling was 41% andsheets to be processed to make can-making steel sheets which wereadjusted to have a tempered hardness of DR8 by work-hardening wereobtained. Sn(tin) was plated on each side of the sheets to be processedin an amount of 2.8 g/m² to finish the sheets to beverage-can-makingcoils. Characteristic values of can-making steel sheets in accordancewith manufacturing conditions are as shown in Table 1 (No. 2).

Each of the coils was laminated with a film except so-callednon-varnished portions. The non-varnished portions were formed parallelto the rolling direction of the coil. Blank sheets for each can unitwere obtained from the film-laminated coil and beverage can barrels weremade by high speed welding at 70 m/min.

After being subjected to a three-stage neck-in processing, each beveragecan barrel was subjected to a flange forming process and thecharacteristic values thereof were tested. Test items were an actualmeasurement of the total thickness of the weld, a measurement of themaximum hardness of the nugget portion by measurement of the microVickers hardness thereof, and the presence or absence of HAZ cracking.HAZ cracking was determined to occur when cans subjected to the aboveflange forming process were crushed so that the welds thereof wereflattened and even a single can of 100 cans was cracked.

The following results from Table 1 were found.

In the case of Example 1 of the invention, since crystal grains had aproper size and the C content was ultra-low, the hardness of the weldednugget portion increased and the total thickness of the weld increasedto 1.3 t and thus no HAZ cracking occurred.

On the other hand, Comparative Examples 2 and 3 contained a large amountof C and had a small grain size as a whole as well as a large totalthickness of 1.5 t to 1.6 t and HAZ cracking occurred. ComparativeExample 4 contained a large amount of Nb and had a large amount ofcrystal grains having a grain size of 5-25 μm due to the large amount ofNb. Thus, the total thickness of the weld was thickened and HAZ crackingoccurred. Although Comparative Example 5 having coarsened crystal grainsdue to an increase in FDT was soft and had a thin total thickness of theweld of 1.4 t, HAZ cracking occurred in the coarse grain boundary. Asimilar result was observed in Comparative Example 6 whose grain sizeswere increased by increasing CT. A similar result was obtained inComparative Example 7 whose grain sizes were coarsened by increasing theannealing temperature in continuous annealing conditions.

                                      TABLE 1    __________________________________________________________________________    (No. 1 CHEMICAL COMPOSITION WT %)    Example       C  Si     Mn  P      S  N        Al Nb    __________________________________________________________________________    Example 1     0.002                     0.03   0.20                                0.015  0.011                                          0.0031   0.015                                                      0.0026    Comparative Example 2                  0.006                     0.03   0.38                                0.014  0.018                                          0.0042   0.091                                                      0.0031    Comparative Example 3                  0.008                     0.01   0.10                                0.013  0.010                                          0.0021   0.152                                                      0.0038    Comparative Example 4                  0.003                     0.03   0.51                                0.015  0.011                                          0.0082   0.078                                                      0.1266    Comparative Example 5                  0.003                     0.03   0.45                                0.015  0.011                                          0.0064   0.078                                                      0.0022    Comparative Example 6                  0.003                     0.03   0.15                                0.005  0.008                                          0.0015   0.083                                                      0.0017    Comparative Example 7                  0.003                     0.15    0.005                                0.008  0.008                                          0.0015   0.083                                                      0.0013    __________________________________________________________________________    (No. 2 MANUFACTURING CONDITIONS, CHARACTERISTIC VALUES)                Hot-Rolling                       Continuous                             Maximum                                   Ratio                                        Total Thickness                                                 Maximum                                                       Occurrence                conditions                       Annealing                             Grain Occupied                                        of Weld mm                                                 Hardness of                                                       of                FDT CT Temp- Size  by 5-25                                        (Ratio to Sheet                                                 Nugget                                                       HAZ    Example     °C.                    C°.                       erature °C.                             μm μm %                                        to be Processed)                                                 Portion HV                                                       Cracking    __________________________________________________________________________    Example 1   840 600                       800   26    85   0.169 (1.3 t)                                                 210   Not occurred    Comparative Example 2                840 610                       750   20    46   0.195 (1.5 t)                                                 210   Occurred    Comparative Example 3                850 630                       750   18    39   0.208 (1.5 t)                                                 222   Occurred    Comparative Example 4                850 650                       750   19    42   0.195 (1.5 t)                                                 168   Occurred    Comparative Example 5                910 650                       750   38    82   0.182 (1.4 t)                                                 171   Occurred    Comparative Example 6                850 700                       750   40    90   0.169 (1.3 t)                                                 173   Occurred    Comparative Example 7                850 570                       820   35    73   0.182 (1.4 t)                                                 173   Occurred    __________________________________________________________________________

What is claimed is:
 1. An ultra-thin and high strength can-making steelsheet excellent in weldability and flange forming processibilitycomprising C≦ about 0.004 wt %, Si≦ about 0.03 wt %, Mn: about 0.05-0.6wt %, P≦ about 0.02 wt %, S≦ about 0.02 wt %, N≦ about 0.01 wt %, Al:about 0.005-0.1 wt %, and Nb: about 0.001-0.1 wt % and the balance Fe,said sheet having a maximum recrystallization grain size of about 30 μmor less, and said sheet having a ratio of area which is occupied byrecrystallization grains having crystallized grain sizes of about 5-25μm, of at least about 50%.
 2. The steel sheet defined in claim 1 whereinthe amount of Mn and S have a Mn/S ratio of about 8 or more.
 3. Thesteel sheet defined in claim 1 wherein said maximum recrystallizationgrain size is about 25 μm or less.